This invention relates to an electrical circuit panel comprising an electrical conductor formed by an electrically conductive coating or coatings on a support, and a lead-in wire connector soldered to the conductor.
Electrical circuits formed on such circuit panels can be of diverse natures and can fulfill different functions. In particular, and by way of example, they can be resistors, inductors, capacitors, antennae, heating panels, or electrooptical panels, or they may comprise a combination of these elements. Alternatively, such a circuit may comprise elements which simultaneously fulfill more than one of these functions.
Such circuits are normally intended to form part of a more complex circuit comprising other elements or electrical apparatus, and to this end the circuit panel is provided with connection means.
A lead-in wire connector is soldered to said conductor in order to ensure maximum conductivity at the joint. Such a connector is usually of a metal, and is often of a different composition from the material to which it is fixed. Under certain conditions, therefore, for example, due to differential thermal expansion or contraction, stresses can build up in the joint, and the joint can deteriorate and even rupture. This necessitates the formation of a new solder joint, which may require removal of the panel since it will not always be possible to re-solder the joint in situ. Under some circumstances an even worse problem may arise in that the conductive coating at the joint may be torn away from the support, and in such cases it is usually necessary to replace the whole panel.
It is an object of the present invention to mitigate this disadvantage.
According to this invention, an electrical circuit panel comprises an electrical conductor formed by an electrically conductive coating or coatings on a support, and a lead-in wire connector soldered to the conductor, and is characterised in that in said conductor there is a discontinuity which exposes an underlying non-solderable surface area, and said connector is in the form of an electrically conductive bridge plate which is soldered to such conductor at two positions spaced apart along such plate on opposite sides of said discontinuity.
Normally, when making a solder joint, the solder will tend to flow along from the positions at which it is applied, and the said discontinuity in the conductor which exposes a non-solderable surface area ensures that even if the solder from such positions should spread across, there will be an area between the said positions where it will not form a bond. Said exposed non-solderable surface area may be an area of the surface of the support, or an area of the surface of a coating applied to the support beneath said conductor.
The present invention also achieves a further advantage, in that by soldering the connector to the conductor at two spaced positions in this way, any internal stresses which build up on the joint will be distributed between the two positions at which solder is applied and the interior of the bridge plate. Because of this, a said joint in a panel according to the invention gives a better resistance to stresses therein whether they arise through differential thermal expansion or externally, for example due to shocks on the panel as a whole or tension, flexure or twisting of the connector itself.
The attachment of a said connector to a said conductor is simplified by making the connector in the form of a plate.
The said conductor(s) of the panel can be formed in different ways and have different structures. For example, an electrically conductive coating constituting a said conductor can be printed onto the support. Such a coating may for example comprise a conductive line or ribbon or a network of such lines or ribbons, or it may comprise a coating layer which might for example be spread substantially uniformly over the greater part or even the whole of the support. By way of example, a conductive coating may be formed by applying to one of the faces of the support by a silk-screen process, an enamel precursor comprising a suspension of metallic particles such as silver and particles of a low melting point glass in an organic binder, and then heating to drive off the organic binder and fuse the enamel. Alternatively, a said conductive coating may be formed by a chemical process, by vacuum evaporation or by pyrolysis.
The conductor deposited on the support can for example be constituted by a single coating or by several superposed continuous coatings of which one or more is a metal or a conductive metal compound.
The support itself can be formed from a body, for example a plate or sheet, of a vitreous material such as glass or a vitro-ceramic, or of a ceramic, or of other suitable insulating material.
The connector is suitably of a highly conductive metal, such as copper or brass.
Preferably, the connector is shaped to provide a laterally extending tongue between said two positions, since this simplifies the attachment of a lead-in wire after the connector has been soldered to a said conductor. The connector may for example be in the form of a T-shaped plate, the two ends of the horizontal portion of the T being soldered to the conductor, and the vertical portion serving as an attachment of a lead-in wire. Such vertical portion of the T may be bent up away from the plane of that portion of the support over which it is soldered if this is desired so as further to simplify the attachment of a lead-in wire.
The connector may, for example, be flat or arched. Embodiments of the invention wherein the connector is flat have the advantage that the connector is less liable to be subjected to knocks, and, if the connector is at an edge of the panel, framing is simplified. Embodiments of the invention wherein the connector is arched between said two positions have the advantage that, because the height of the center of the arch above the underlying surface is greater than that of the ends of the arch, the tendency of solder applied at said two positions to flow along under the bridge plate is reduced; also, the connector itself can flex more easily under the influence of differential thermal expansion of the connector and the support, thus reducing strain on the joints.
Preferably, the discontinuity in the conductor is at least 6mm long measured in the direction between said two positions. This also tends to limit solder flow beneath the connector plate. It is preferred that the non-solderable surface area beneath the plate should extend across the whole width thereof, thus completely separating the two soldering positions. Of course, any solderable area beneath the plate should be small enough and/or sufficiently well spaced from the two soldering positions as to inhibit the flow of solder therealong. It is especialy suitable to form the conductor to which the connector is soldered as a strip which is interrupted along its length. Such interruption then constitutes the discontinuity which exposes said underlying non-solderable surface area. The portions of such a conductor strip on opposite sides of the interruption may be formed simultaneously or successively. In some cases it may be desired to cause different currents to flow in such different conductor portions, and this can readily be done either by altering the cross-sectional area of one strip portion with respect to the other, or by forming the two strip portions of different materials.
Preferably, the conductor to which the connector is soldered is in circuit with at least one other conductor of higher resistance. In this way, the feeding of current to or from the circuit carried by the electrical circuit panel is made more effective. Electrical circuit panels according to the present invention in which this feature is adopted are especially suitable for use as resistance heating panels.
In one preferred form of electrical resistance heating panel according to the invention, there are two said conductors to which lead-in wire connectors are soldered, and said conductors are interconnected by a plurality of electrically conductive coating strips of higher resistance than said two conductors. This is a very convenient way of ensuring that that portion of the support which carries conductive material is substantially uniformly heated. Such higher resistance coating strips may be of the same material as said two conductors, and may be applied to the support by or in the same process. For example, higher resistance coating strips and said conductors may be of a conductive enamel and applied simultaneously be a silk-screen process. In order to achieve their higher resistance in such a case, those coating strips would of course have to have a smaller cross-sectional area than said conductors.
In another preferred form of electrical resistance heating panel according to this invention, there are two said conductors to which lead-in wire connectors are soldered, and such conductors are deposited on top of another conductor of higher resistance which is constituted as a substantially uniform electrically conductive coating layer. Such a coating layer may for example be formed by electro-deposition or pyrolysis and extend over substantially the whole of one surface of the support. One of the differences between these two types of heating panels is a matter of aesthetics. For example, in the case where such panels are transparent and utilized as heatable rear windows of motor vehicles, for which use they are especially suitable, one user may prefer to suffer the slight loss of overall transparency given by a uniform coating layer rather than have the window crossed by a series of coating strips, whilst another user may prefer high transparency over the greater part of the window and accept the presence of the coating strips. However, in the case of heatable vehicle windscreens where such coating strips might prove visually distracting, it is preferable to have a uniform transparent coating layer.
Preferably, the maximum height of a said connector above the support is less then 3mm. This reduces the likelihood of the connector being jolted or otherwise subjected to mechanical shocks, and where such connector is located at the edge of the panel, it simplifies framing thereof should that be required.